Cagliari is beautiful and not as overrun with tourists than other Italian cities.
It's amazing to be here for the Feast of Sant'Efiseo (May 1)--it kicks off with a fantastic parade through the historic center--natives in traditional costumes, flower-covered carts, soulful music played on launeddas=Sardinian bagpipes.
Cagliari in Sardinia is warm destination all year (i did last swim on 3 November) and it's really a great fun; the city centre - Marina & Castello - is pure movida on night, local restaurant, plenty of attraction and live gig (rock & theatre). It still a cheap destination where you can eat for resonable price. What else, for tripadvisor, on 2012 it gain 4th position on beach destination in all Italy, so you can read different review and since Raynair & Easyjet is landing here it's full of tourist (not crowded like florence). If you like also small hiking (like trekking) there are beautiful walking up to "The Sella Del Diavolo" and many more things to do!
70% of Cagliari was destroyed during the First World War. it is not hard to imagine it as, apart from the Castello area and some other places, there are many buildings of 30s and after. Cagliari has around 160.000 inhabitants and lays directly on the sea. there is a nice beach that is part of the city itself. there are many interesting bars and spots, mainly in Castello area, as far as we could sea. a full day is enough however, in my opinion to explore it.